1. Field of the Invention
The present invention relates to a ceramic clay, a ceramic formed article, and a ceramic structure, and manufacturing methods thereof.
2. Description of the Related Art
Ceramic products are usually manufactured through methods such as jiggering, extrusion, injection molding, press molding, and sheet forming. However, ceramic raw material powders, when used alone, cannot satisfy plasticity, shape retention property, and the like necessary for manufacturing ceramic products. Therefore, after adding water, an organic binder, and the like to the powders, the resulting ceramic forming raw material is molded or formed into ceramic products. For example, in extrusion, there is disclosed a manufacturing method of a ceramic structure which includes extruding a forming raw material (clay), which has been obtained by kneading a ceramic raw material, water, an organic binder, and the like, and therefore has improved plasticity, drying and firing the extrudate (refer to, for example, Patent Document 1).
As the amount of an organic binder to be added in order to provide ceramics with plasticity and shape retention property becomes greater, the resulting ceramics have improved moldability or formability. For example, when a large-sized structure or a structure having a complex cell structure, the demand for which is increasing in recent years, is manufactured, a kneaded clay (clay) having good moldability or formability becomes more necessary than that in the manufacture of a small-sized structure or a ceramic structure having a simple cell structure. As a result, a large amount of an organic binder has to be added.
However, when the amount of the organic binder is large, a space occupied by the organic binder becomes a defect at the time of molding or forming because the organic binder disappears upon firing, leading to deterioration in mechanical strength of the resulting ceramic structure. In a large-sized structure, there has been a problem that the temperature inside the structure becomes higher than that on the external surface of the structure due to burning heat when the organic binder burns during the firing and defects such as cracks appear because of a thermal stress due to an inner/outer temperature difference of the structure. It not only deteriorates the mechanical strength of the structure but also greatly reduces a production yield. Furthermore, a CO2 or harmful gas generated and released to the atmosphere after burning of the organic binder during the firing has raised a problem in environmental respects such as air pollution and global warming.
With a view to overcoming such problems, Patent Documents 2 and 3 disclose a ceramic clay containing a layered double hydroxide such as hydrotalcite. The layered double hydroxide such as hydrotalcite is, similar to the organic binder, also effective for enhancing the fluidity of the clay and improving the moldability or formability. Addition of a layered double hydroxide therefore enables reduction in the amount of the organic binder, making it possible to suppress the problems such as generation of defects or harmful gas upon firing which will otherwise occur when the amount of the organic binder is large.
The amount of the layered double hydroxide necessary for reducing the amount of the organic binder and thereby overcoming the above problems is however as large as about 6 mass %. A ceramic structure manufactured using a ceramic clay containing such a large amount of a layered double hydroxide inevitably has other problems such as an increase in the manufacturing cost and deterioration in product properties such as increase in a thermal expansion coefficient.    [Patent Document 1] Japanese Patent No. 3227039    [Patent Document 2] WO 2005/095302    [Patent Document 3] WO 2008/044497